N2-fixing cyanobacteria are one of the microorganisms that hold a role in the cycles of nitrogen in a rice field. The existence of free-living N2-fixing cyanobacteria in paddy fields provides advantages for rice crop. The diversity of N2-fixing cyanobacteria has been extensively studied in the conventional rice field, while so far, there is no record of cyanobacterial occurrence at organic rice field. At present, implementation of organic rice-system has been increasing. Organic rice field is promoted as an alternative way of keeping the health of paddy field. From that point of view, investigation of N2-fixing cyanobacteria as a part of soil components is important. This study investigated the diversity of the filamentous N2-fixing cyanobacteria assemblage in the organic rice field during the cycle of rice crops (from Dec 2016 to July 2017; approximately two crop cycles). Soil samples were collected from four plots of 20 ha rice field. At each plot, sample soils from three random stations were taken using 3 cm diameter plastic cylinder and mixed to composite soil samples. The result showed that the number of species was low at the beginning and the harvest time of rice plantation (10 and 11, respectively). The number of species reached a peak at the panicle initiation (20 species). Three species were always found at all stages of rice crop's cycle: Nostoc punctiforme, Nostoc muscorum, and Anabaena fertilissima. Genus Nostoc was dominant followed by Anabaena, Calothrix, Cylindrospermum, Nodularia, Scytonema, Trichormus, and Stigonema.